1. Field of the Invention
The present invention relates to a process for the purification of crude 2-mercaptobenzothiazole.
2. Description of the Prior Art
2-Mercaptobenzothiazole and its derivatives such as dibenzothiazole disulfide, cyclohexylbenzothiazylsulfenamide, N-oxydiethylenebenzothiazyl-sulfenamide, zinc salts of 2-mercaptobenzothiazole, diethylbenzothiazyl sulfenamide and the cyclohexylamine salt of 2-mercaptobenzothiazole which are generally referred to in the vulcanization art as the thiazole series of accelerators are important vulcanization accelerators which are used in large quantities in the natural and synthetic rubber industry. Moreover, many patents and literature references have been published on the preparation and purification of 2-mercaptobenzothiazole.
It is well known that in the conventional Kelly process for preparing 2-mercaptobenzothiazole by the reaction of aniline, carbon disulfide and sulfur under elevated pressure at high temperatures, the product contains not only by-products such as benzothiazole, anilinobenzothiazole, phenylisothiocyanate, benzoisonitrile and a tarry material of unknown composition, but also unreacted sulfur, aniline and carbon disulfide. These by-products and unreacted materials are usually removed from the product by dissolving it in a dilute solution of caustic soda, and filtering the solution after partial oxidation and acidification if necessary. However, in this process a large amount of water is used to dilute the caustic soda solution, it takes many hours to remove suspended substances from the solution to form a clear solution of the salt of 2-mercaptobenzothiazole sodium, and the tarry matter which separates from the solution is so dirty and foul smelling that it is very troublesome in handling.
One effort to improve the purification of crude 2-mercaptobenzothiazole involved the extraction of the reaction products with carbon disulfide. The method, as disclosed in Canadian Pat. No. 448,209, is based on the fact that all of the above described impurities are very soluble in carbon disulfide, while 2-mercaptobenzothiazole is less soluble. However, this method must be conducted in apparatus which is relatively small compared to the apparatus employed in the caustic soda process. Moreover, the technique is also used as a method for treating solid crude 2-mercaptobenzothiazole. The technique, has the disadvantage of being more expensive than the conventional liquid extraction process since it must be conducted in batch type systems requiring much labor and time. In addition, the method is detrimental to human health because of gases evolved in the extraction of crude 2-mercaptobenzothiazole which include small amounts of hydrogen sulfide. Moreover, as is evident from the disclosure of U.S. Pat. No. 2,090,233, in the extraction of solid, crude 2-mercaptobenzothiazole with carbon disulfide, a large amount of solvent is required to achieve the desired purity level of 97.5 to 99.5%, since the extraction process must be repeated two or three time or more to completely remove the tarry impurities.
In summary, the prior art procedures for purifying crude 2-mercaptobenzothiazole have many deficiencies in that a large amount of solvent or water is consumed, it is difficult and troublesome to separate and handle the tarry by-products, much labor and time are required and they can be detrimental to human health.